btcd before 0.24.0 does not correctly implement the consensus rules outlined in BIP 68 and BIP 112, making it susceptible to consensus failures. Specifically, it uses the transaction version as a signed integer when it is supposed to be treated as unsigned. There can be a chain split and loss of funds.
Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B’s state.